Cholesterol must be good for us, because it's thecommonest steroid in animal tissues, where it makes cellmembranes. Yet for more than a decade now, people haveput cholesterol on their dietary least-wanted list.
Everyone now knows that cholesterol is bad for us,because it causes build-up of atherotic plaque, whichcauses atherosclerosis in coronary arteries, which causesischemic heart failure, which causes death.
This potentially fatal "hardening of the arteries" tends tostrike people in their 50s and up. Its visible signs in manyolder people are the yellowish, blister-like pouches full ofcholesterol under eyelids, and on knuckles, elbows andknees.
These cutaneous xanthomas are a hallmark of familialhypercholesterolemia, (FH), which explains why some _a mercifully few _ children develop xanthomas astoddlers. "This is why FH is most often first identified bydermatologists," observed gene therapist MariannGrossman, who focuses on this disease.
"One small patient seen first by a dermatologist," sherecalled, "had a tiny xanthoma on her knee. Hercholesterol level was 650 milligrams per deciliter _ overfour times the normal average."
Some infants so afflicted may die of heart disease by theirsecond year of life; most will by their 20s. They are thevictims of a mutant gene that causes FH. Each of theirparents carries a single copy of the aberrant DNA, whichmeans they are heterozygotes. So one in four of theiroffspring stands to inherit both the maternal and paternalmutant genes, a double dose that makes themhomozygotes.
Those mutations clobber the cell-surface receptor forlow-density lipoproteins (LDL), the carriers that ferrycholesterol from its site of manufacture in the liver to thezillions of cells that forever need new membranes. LDLsare the "bad" lipoproteins that keep cholesterol trappedon an endless circuit within the body. High-densitylipoproteins (HDL) are the "good guys" that hustle theharmful lipid out of the body, by way of the liver.
Because each of an FH homozygote's two defective genesresults from a single point mutation that knocks out theirLDL receptors, such patients make ideal candidates forgene therapy.
Gene therapist James Wilson, now at the University ofPennsylvania's Institute for Human Gene Therapy, inPhiladelphia, together with Grossman, lined up the firstof five homozygous FH candidates in March 1992. (SeeBioWorld Today, April 4, 1994, p. 1.) "Patient # FH-1"was a French-Canadian woman of 28 from Quebec.
FH-4 was also a French-Canadian woman, 41 years ofage. FH-2 and 3 were pre-teen children, Greek andColombian respectively. And FH-5, a Chinese girl of 11,had come from Beijing in March 1994, referred to Wilsonby her physicians there for the trailblazing FH genetherapy trial.
An article in this month's Nature Medicine chroniclesthat effort, under the title: "A pilot study of ex vivo genetherapy for homozygous familial hypercholesterolemia."
Grossman and Wilson, the paper's first and senior authorsrespectively, based their strategy on promising preclinicalexperience in a line of Japanese FH-model white rabbitsby the strain name of Watanabe.
Because LDL receptors _ the hapless victims ofhomozygous FH mutation _ stud the surface of livercells, the gene therapy team removed 10 to 20 percent ofthe rabbits' livers, and teased out their receptor-lackingcells. They exposed these cells, in culture, to infection byrecombinant retroviral (RV) vectors that carried atransgene encoding the LDL receptor protein.
After the team engrafted this package in a vein that fedthe rabbits' liver, the co-authors reported, "thehepatocytes expressing the transgene [reconstituted]approximately five percent of normal LDL receptorexpression. This translated to a 20 to 40 percent decreaseof serum cholesterol, which was stable at least 4.5months."
Thus encouraged, the gene therapists extrapolated this exvivo animal paradigm to their five FH patients. Theirtherapeutic outcomes, though partly successful, missedliving up to the results promised by the Watanabe FHmodels.
Inbred Rabbits Out-Do Inbred People
As the Nature Medicine paper noted, "Although genetransfer was demonstrated in all patients, efficiencieswere low . . . and exhibited substantial patient-specificvariation . . . Heterogeneous responses to gene therapyshould be anticipated whenever one attempts to translatepreclinical [animal] data to the extraordinarily diversehuman population."
"All five patients are still alive," Grossman toldBioWorld Today on Friday, "still taking cholesterol-lowering drugs in addition to their gene-therapyimprovement." The French-Canadian FH-1 and ChineseFH-5 patients have maintained the lowered cholesterolcounts that followed their gene therapy. The former's,Grossman said, is down 23 to 24 percent; the latter's,around 20 percent.
With this final report in Nature Medicine, Wilson andGrossman have closed the chapter, but not the book, ontheir ex vivo FH gene therapy. For their follow-on FHprogram, they have set up a separate in vivo drawingboard.
Grossman recently moved from Wilson's Institute tobecome president of a spin-off biotechnology company,Genovo Inc., also in Philadelphia. (See BioWorld Today,Aug. 16, 1995, p. 1.)
Here is how she sees the near-term future for their still-joint FH gene therapy: "Jim Wilson's group," she said,"as well as we at Genovo, have really made a lot ofprogress toward the in vivo approach, using second-generation adenoviruses. As vectors," Grossmancontinued, "the adenoviruses appear to be more efficientin livers than the RVs were in our ex vivo study."
Meanwhile, the Philadelphia team is collectinghomozygous FH candidates for their future in vivo trials."Right now we're up to about 40 patients," Grossmansaid. "They come to us once a year, get a completecardiac work-up, including catheterization, so we canfollow their progression of coronary disease over time.The youngest is 15 months of age, the oldest that FH-4woman, now 42, from Quebec province." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.